I can think of three ways.
Case 1: You know the subnet, and the SLC-5/05 is new enough to support EtherNet/IP.
The EtherNet/IP driver browses by sending out a broadcast packet with the "List Identity" service request. All devices that get the message reply with their identity information.
Case 2: You know the subnet, but don't know the IP address.
Use the freeware NMAP port scanner to check each device on your subnet for an open TCP Port 2222.
Case 3: You don't know the subnet, but you have a PC without a firewall.
Use the BOOTP/DHCP Server utility that comes with RSLinx to send an "enable BOOTP" request to the controller's MAC ID.
If none of these are the case, you're going to have to default the controller and let it go back to BOOTP mode.